The intervertebral disc (IVD) permits articulation between adjacent elements of the spine. The disc includes an outer annulus fibrosis and an inner nucleus pulposus. In a healthy disc, the nucleus is a gel that transmits load and absorbs shock. Loads are constrained axially by the annulus fibrosis. Through degenerative processes and/or trauma, the annulus may fail and release the nucleus, which is then free to flow.
The posterior annulus is typically thinner than the anterior annulus, thus, making failures of the posterior annulus more common. When these failures occur, a variety of problems may arise. For example, the contents of the disc may impinge onto nerve roots and/or the spinal cord, resulting in pain and/or neurological deficits. IVD failures in the lumbar region of the spine are most common, but failures can occur at any level.
When disc failure occurs and pain is present, discectomy may be indicated to remove the impinging material. A 5-10% recurrence of painful extrusion may occur. Loss of the nucleus leads to kinematic changes to the segment and can accelerate weakening of the annulus and development of osteophytes at the vertebral endplates. This development of ectopic bone may lead to stenosis of the vertebral canal and detrimental changes to other articulating elements.
Injected liquids and implants have been proposed for insertion into a spinal disc space for different purposes. For example, some proposed systems include inflated balloons to distract disc space to restore lost height as preparation for an injected biomaterial or as an implant. Other systems include injecting liquids that harden or thicken in situ or hydrogel systems that expand upon exposure to water. The use of such devices, however, shares a common weakness, in that a failure of the device would tend to lead to rapid expulsion of the filler material, for example, through a pre-existing defect in the annulus. Such expulsion may create impingement in the same area that created a need for surgical intervention in the first place. For example, failure of such a device may lead to loss of material from the disc space which may then impinge on neural elements. The loss of any material from such devices may lead to loss of disc height and detrimental changes to segment kinematics. Further, the arrangement of many of these devices would tend to lead to expulsion of the device itself, if deflated. Additionally, for devices constructed from a single chamber, the uneven compression of the disc in flexion and extension can result in improper loading of the disc space.